This invention relates to an optical system and, more particularly, to an optical system with a boresight source that is used to establish the centroid of a sensor imager.
In one type of optical system, a telescope directs the light from a scene to a photosensitive device such as a focal plane array (FPA). The light may be of any suitable wavelength and is typically in the visible and/or infrared ranges. Some optical systems utilize two different wavelength ranges, such as the visible and the infrared. The FPA converts the incident light into electrical signals, which are then processed electronically in a tracker for viewing or automated image analysis.
In order to determine the location of the image relative to the plane of the FPA, a boresight source is provided. The boresight source creates a uniform boresight light beam at the FPA so that the tracker portion of the optical system may precisely locate the centroid of the FPA. The image of the scene is then related to this precisely located centroid.
The boresight accuracy and thence the accuracy of the optical system are determined by several factors, including the uniformity of the boresight light beam and the temperature difference between the boresight source and the background. The boresight source must therefore generate the boresight light beam with high spatial uniformity. The boresight light source produces a light beam that is somewhat nonuniform. In conventional practice, the boresight light beam is directed through a pinhole to improve its spatial uniformity. The size of the pinhole is often limited to a few thousandths of an inch in diameter to achieve the desired beam spatial uniformity. Consequently, the beam passing through the pinhole does not have sufficient brightness and signal-to-noise ratio to provide the required boresight accuracy.
Although a coherent light source such as a laser diode may be used to increase the brightness, the beam uniformity is greatly degraded due to the speckles associated with a typical coherent light source. One way to achieve the uniform beam is to employ a pinhole with a diameter around one-half of the size of the Airy disk, which is typically about 10 to 20 micrometers for the visible and near-infrared wavelength. Most of the energy of the light source does not pass through this small pinhole and is lost. Additionally, it is quite difficult to fabricate a highly precise pinhole of this small a size suitable for use in the boresight source. The result of using an imprecise pinhole is that the spot of radiation on the FPA is not uniform, and the accuracy of the tracker is degraded. The small pinhole also leads to a low efficiency and a low signal-to-noise ratio.
There is a need for an improved approach to the boresight source, which allows the optical system to maintain high accuracy even for operation in the visible and short-wavelength infrared ranges. The present invention fulfills this need, and further provides related advantages.
The present invention provides an optical system which includes an extended boresight source. The boresight source produces a beam which is highly spatially uniform and collimated, even for operating wavelengths in the visible and short-wavelength infrared ranges. The boresight source of the invention does not require any change in the structure and operation of the remainder of the optical system, which may be optimized for its performance.
An optical system having an extended boresight source comprises a boresight light source that produces a light beam, a condenser lens that receives the light beam from the boresight light source, a spatial light integrator that receives the light beam from the condenser, a constriction through which the light beam from the spatial light integrator is directed, and a collimator that receives the light beam which passes through the constriction and outputs a boresight light beam. The optical system usually further includes a sensor imager that receives the boresight light beam from the collimator and uses the boresight light beam for locating and alignment purposes.
The boresight light source preferably emits light in the wavelength range of from about 0.4 to about 12 micrometers, and is preferably a light bulb. In some applications, the light source may be a laser diode, whose driving voltage or current may be modulated to achieve temporal incoherence. The spatial light integrator may be a light pipe, such as a refractive rectangular light pipe or a hollow reflective rectangular light pipe. The spatial light integrator may instead be a combination of a lens array that receives the light beam from the condenser lens and a focusing lens that receives the light beam from the lens array. The constriction may be a field stop or a pinhole, for example.
The approach of the invention produces a highly spatially uniform boresight light beam even though the boresight light source may be somewhat nonuniform. The centroid of the light sensor may therefore be located very accurately, with a corresponding high accuracy of the tracker of the optical system. The present approach does not depend upon diffraction effects to achieve a uniform boresight light beam, and is accordingly readily implemented in practice and is optically efficient. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.